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Handbook of Size Exclusion Chromatography and Related ...

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Figure 20 SEC chromatogram for an asphalt using a multiple-wavelength UV detector.<br />

(From Ref. 154, p. 172.)<br />

result <strong>of</strong> different carbon–hydrogen ratios in the materials. The differences in<br />

response were comparable to those in RI or UV detectors. These detectors are<br />

generally more expensive <strong>and</strong> more difficult to operate than RI or UV detectors,<br />

however.<br />

Another evaporative on-line detector is the evaporative light-scattering<br />

detector (ELSD) (152,160–164). In the ELSD, the eluant is nebulized with an<br />

inert gas to form an aerosol. The solvent in the dispersed eluant droplets is<br />

evaporated <strong>and</strong> removed in a heated chamber. The resulting solute particles fall<br />

through a light-scattering detector. The scattered light is related to the amount <strong>of</strong><br />

mass in the particles, which in turn corresponds to the amount <strong>of</strong> solute in the<br />

eluant.<br />

The light scattering is supposed to be minimally dependent upon the<br />

structure <strong>and</strong> functionality <strong>of</strong> the solutes. The sparse literature pertaining to asphalt<br />

<strong>and</strong> heavy petroleum fractions indicates that the detector’s response varies with<br />

different solutes, however. Pentane solubles gave markedly lower response than<br />

asphaltenes <strong>and</strong> benzene insolubles. The response to pentane solubles also varied<br />

with evaporator temperature, which is usually a sign <strong>of</strong> solute loss by evaporation.<br />

This seems unlikely with a material as nonvolatile as asphalt. Like the evaporative<br />

FIDs, the ELSD is more expensive <strong>and</strong> more difficult to operate than the RI or UV<br />

detectors.<br />

© 2004 by Marcel Dekker, Inc.

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